Rarely epidote occurs in crystals, which belong to the monoclinic system, and may be either short like the diagrams on plate 42 or long and needle-like. The color and hardness will suffice to determine this mineral, as almost no other has the peculiar yellowish-green color which is characteristic of this form. Epidote occurs primarily in metamorphic rocks at or near the contact with igneous rocks; or it may be a secondary mineral resulting from the weathering of granites, especially along seams. It sometimes occurs with hornblende in highly folded schists, as in New York City. It is often a mineral which has resulted from the alteration of other minerals, as pyroxene, amphibole, biotite, or even feldspars. It is found at Chester and Athol, Mass., Haddam, Conn., Amity, Munroe and Warwick, N.Y., East Branch, Penn., in the Lake Superior region, in the Rocky Mountains, etc. Tourmaline (FeCrNaKLi)₄Mg₁₂B₆Al₁₆H₈Si₁₂O₆₃ Pl. 42 & frontispiece Occurs in three-sided prismatic crystals; hardness, 7; specific gravity, 3.1; colorless, red, green, brown, or black; luster vitreous; transparent on thin edges. Tourmaline is readily distinguished from other minerals, as it always occurs in long to short prisms, which are three-sided in cross section. There is also a tendency for the sides to be curved as seen on the end view of D, Pl. 42. Frequently the vertical edges of the prism are beveled with one, two or three faces, grouped about each of the three original edges, and there are often striations on the prism faces. The ends are terminated by a low rhombohedron and again there may be a host of modifying faces on the edges and corners of the end. The common varieties are brown or black in color, but occasionally there may occur green, red, yellow or almost any color. When the crystals are perfect, that is free from impurities and without tiny cracks, tourmaline becomes a gem of popularity and value. Tourmaline is very complex in composition and may vary considerably, the sodium, potassium, lithium, magnesium, and iron being either more or less abundant or even lacking. The color is to some extent dependent on the proportions of these elements present, the dark varieties having more iron, and the light colored tourmalines lacking it. This mineral is one of those which form from superheated vapors, escaping from molten magmas. It will therefore occur in veins, often associated with copper minerals, in crystalline limestones, or in cavities in granites, where it is associated with such minerals, as beryl, apatite, fluorite, topaz, etc.